What causes a tractors front end loader to raise up on its own?

Year, make, and model would help, as I'd be able to look up the hydraulic diagram. Typically, that's going to be a valve which doesn't close fully. Depending on exactly what you have, it could simply be a worn valve, or, if it's actuated by an electrical solenoid, there could be an issue with that solenoid. Another possibility (although you'd notice it in areas other than the bucket, as well) is excess system pressure (often caused by a restriction), but you'd most likely be blowing out hydraulic lines if this were the case.